1. The Field of the Invention
This invention relates to improved compositions and methods for priming or sealing dental substrates and biological substrates, particularly exposed dentinal tooth surfaces, in order to obtain strong adhesion when followed by an appropriate material such as a resinous material or glass ionomer cement. More specifically, the invention relates to improved compositions and procedures for applying an approximately pH neutral dentin primer requiring no mixing by the applicator as it is a highly stable one component primer system.
2. The Prior State of the Art
Resinous materials are widely used for a variety of dental uses, including cavity filling, cosmetic enhancement and the cementation or adhesion of resinous and non-resinous materials.
Because tooth enamel is comprised mostly of calcified minerals, it is very hard and can be physically cleaned and chemically etched (conditioned) in preparation for bonding with resinous materials. Once dried by air, and/or using a drying agent, the enamel tends to remain relatively dry. After the enamel has dried, hydrophobic type resins can be bonded on to the enamel. This bond between dried enamel and hydrophobic resins as well as the process of obtaining such bonds are possible primarily due to the inorganic mineral content of enamel which is very high compared to that of dentin.
In sharp contrast to the process involved in bonding resinous materials to enamel, it is much more difficult to obtain completely dry dentin by using a drying agent and/or air drying techniques. This is because dentin contains thousands of dentin tubules which radiate outward from the pulp of the tooth and which contain pulp fluid under pressure. A cross-section of dentin contains approximately 35,000 dentin tubules per square millimeter, which can ooze fluids from the pulp after the dentin surface is cut or drilled and the smear layer has been altered or removed.
Cutting or drilling the dentin creates a "smear layer," which is a semi-attached, weakened layer of dentin. Tests have shown that it is difficult to obtain strong bonds between resinous materials and dentin unless the smear layer has been substantially removed. This is usually accomplished by the use of aqueous acid solutions or chelating agents.
Another obstacle to obtaining strong bonds between resinous materials and dentin is that dentin is relatively soft and less durable than enamel. While enamel has a mineral content over 97% comprising mainly calcium hydroxyapatite, phosphates, etc, dentin comprises only about 45% mineral material--the other 55% being a fibrous protein matrix and water. Dentin, therefore, has a hardness, strength and chemical composition more similar to bone tissue.
Not only is enamel easier to dry and to keep dry compared to dentin, it can be more predictably etched in order to yield a superior bonding surface. Chemical etching, typically by means of a moderately strong acid (such as 30-40% aqueous phosphoric acid) creates microscopic irregularities and undercuts into which a resinous material can enter, thereby providing a strong, reliable bond to the etched enamel.
The first attempts to improve dentin bonding involved trying to implant resinous "tags" within the cores of the dentin tubules. This mechanism, developed by Japanese scientists in the 1970's, postulated that resinous materials could form reliable bonds via these tags. However, due to the relatively small cross-sectional area of the dentin tubules compared to the dentin surface, and because resins shrink upon polymerizing, and due to the pulp fluid within the dentinal tubules, the resinous tags did not adhere well to the dentin. Hence, this method did not result in acceptably strong and durable bonding to dentin.
During this time and after, the theory among dentists and dental researchers in the U.S. was that it was not desirable to remove the entire smear layer, although it later became understood that any resulting bonds could be no stronger than the bond of the smear layer to the dentin.
Later generation formulations generally reflect the teachings of Rafael L. Bowen and others. These achieve bond strengths of around 14-20 MPa. A number of patents generally reflect this later school of thought, including, e.g., U.S. Pat. No. 4,588,756 to Bowen; U.S. Pat. No. 4,659,751 to Bowen; and U.S. Pat. No. 4,964,911 to Ibsen et al. Bowen '756 teaches a three step process which includes the following steps: first, applying a polyprotic acid; second, applying a solvent containing at least one compound selected from the group consisting of N-phenylglycine, the adduct of N(p-tolyl)glycine, and glycidyl meth-acrylate (hereinafter "NPG-GMA"), and the addition reaction product of N-phenylglycine and glycidyl methacrylate (hereinafter "NPG-GMA"); and third, applying a solution containing at least one compound selected from the group consisting of the addition reaction product of pyromellitic acid dianhydride and two hydroxyethyl methacrylate (hereinafter "PMDM"), the addition reaction product of 3,3',4,4'-benzophenonetetracarboxylic dianhydride and 2-hydroxyethylmethacrylate (hereinafter "BTDA-HEMA") and 4-methacryloxyethyl-trimelliticanhydride (hereinafter "4-META").
Bowen '751 teaches a two step process which includes the following steps: first, applying an aqueous acid solution including strong acids, acids containing polyvalent cations, and acids which form precipitates with calcium, and a compound selected from the group consisting of N-phenylglycine, NTG-GMA, and NPG-GMA; and second, applying a solution containing either PMDM, BTDA-HEMA or 4-META.
Ibsen '911 teaches a similar three step process which includes the following steps: first, applying an acidic solution containing a variety of strong and polyprotic acids; second, applying a solvent containing a variety of different resins or amino acids; and third, applying a solution of PMDM.
A number of these later generation commercial dentin bonding agents are sold, including All Bond II (manufactured by Bisco Dental Products), Prisma Universal Bond III (manufactured by Dentaply), Tenure (manufactured by Den-Mat Co.), and Multi-Purpose (manufactured by 3M).
Dentin primers are now available which do not require mixing, such as, Opti-Bond from Kerr and Prime and Bond from Caulk Dentsply. Although, such primers and/or bonding agents can be more quickly used than primers requiring mixing at the time of application, there are disadvantages which minimize their usefulness. One of the primary disadvantages of these primers is related to the need for multiple coats. The efficiency gained by using premixed primers is lost by the need for multiple coats to obtain adequate coverage and polymerization. The economic usefulness of the primers is also limited since the shelf life of such dentin primers is typically very short despite the claims of the manufacturers. It is well known in the industry among prominent lecturers and clinicians that most of the primers have a significantly shorter shelf life than claimed by the manufacturers. Additional problems may result from the highly acidic nature of such primers. The low pH is potentially harmful since the polymerized primer may leach and since a residual portion of the primer is typically not polymerized thereby remaining acidic and in close contact with the dentin. A residual unpolymerized portion could also potentially cause continued degradation to the remaining tooth structure. Low ph may also cause potentially high tooth sensitivity and may even cause painful injury if the acidic primer comes into contact with the pulp of a tooth underlying the exposed dentin.
Additionally conventional primers when applied to a biological substrate as a sealant before application of cementation materials such as glass ionomer cement, polycarboxylates and the like usually to interfere with the ability of cements to bond. Inadequate bonding strengths usually result when a conventional primer is applied to a metal, enamel, porcelain or composite and then followed by a bonding resin.
In view of the foregoing, it would be an advancement in the dental art to provide compositions and methods for more efficiently obtaining stronger, more predictable bonds between dentin and subsequently placed resinous materials. It will be appreciated that it would also be a significant improvement over the prior art to provide dental compositions having a long shelf life which would result in dependable bonds over time during storage and usage
Further, it would be a significant improvement if the dentin bonding primer was stable as one component and more stable than all types of existing dentin primers.
It would be an improvement over the prior art to provide compositions and methods for sealing a biological substrate without interfering with the bond achieved with a material which is thereafter applied such as glass ionomer cement, carboxylate cements or the like.
Further, it would be a significant advancement in the dental art to provide composition and methods for a single component primer which has an approximately neutral pH level.
Finally, it would be an important advancement if such compositions and methods could provide significant bonding strengths to an appropriately conditioned enamel, porcelain, metal or composite surface(s) when followed by an appropriate resin.
Such compositions and procedures for achieving these results are set forth and claimed herein.